June 30. Ether is superfluous: Faraday and Einstein

On this day in 1905 Albert Einstein’s paper, “On the Electrodynamics of Moving Bodies,” arrived at the editorial offices of the journal Annalen der Physik. The paper was published three months later, and the theory later became known as special relativity. Physics Today.

Faraday began saying: “I incline to believe that when there are intervening particles of matter (being themselves only centers of force), they take part in carrying on the force through the line, but when there are none, the line proceeds through space… we can at all events affect these lines of force in a manner which may be conceived as partaking of the nature of a shake or lateral vibration…
It may be asked, what lines of force are there in nature which are fitted to convey such an action and supply for the vibrating theory the place of the aether?” The lines of force could be electrical, magnetic or gravitational. “…all I can say is, that I do not perceive in any part of space, whether (to use the common phrase) vacant of filled with matter, anything but forces and the lines in which they are exerted…
The view which I am so bold to put forth considers, therefore, radiation as a high species of vibration in the lines of force which are known to connect particles and also masses of matter together. It endeavours to dismiss the aether, but not the vibrations”.
In addition, Faraday explains that the vibrations that occur on the surface of disturbed water, or the waves of sound in gases or liquids, are “direct or to and fro from the centre of action”, whereas the (radiant) vibrations are “lateral”.
Faraday then explains the problem that is inherent in an ether model. He writes that it seems to him, “that the resultant of two or more lines of force is an apt condition for that action which may be considered as equivalent to a lateral vibration; whereas a uniform medium, like the ether does not appear apt, or more apt than air or water”.

Faraday’s lines of force inspired Maxwell in developing electric and magnetic field theory, but Maxwell did not follow Faraday in rejecting the ether. Maxwell developed mechanical models for the ether.

Einstein, apparently not knowing about Faraday’s 1846 rejection of the ether, started his 1905 relativity paper with the problematic asymmetries that were inherent in the electrodynamical explanation of the phenomenon of induction by Faraday. Hence, Einstein started with Faraday.

According to Faraday, when a magnet and a closed electric circuit are in relative motion, an electric current is induced in the electric circuit. This current is actually a result of the relative motion between the magnet and the conductor.
If the conductor is at rest in the ether and the magnet is moved with a given velocity, a certain electric current is induced in the conductor. If the magnet is at rest, and the conductormoves with the same relative velocity, a current of the same magnitude and direction is in the conductor. The ether theory gives a different explanation for the origin of this current in the two cases. In the first case an electric field is supposed to be created in the ether by the motion of the magnet relative to it (Faraday’s induction law). In the second case, no such electric field is supposed to be present since the magnet is at rest in the ether, but the current results from the motion of the conductor through the static magnetic field.

This asymmetry of the explanation is foreign to the phenomenon, because the observable phenomena (the current in the conductor) depend only on the relative motion of the conductor and the magnet.

Parts of Faraday’s talk are also quoted in “Fields of Force: The Development of a World View from Faraday to Einstein”, By William Berkson, pp. 97-99.